In recent years, speech coding research has focused on reducing data width of speech signals while retaining voice quality, signal strength (robustness) under noisy conditions, and minimizing both coding delay and complexity. Vocoders have been developed to transmit speech signals in a coded manner to reduce the transmission bandwidth which would otherwise be required. As disclosed in U.S. Pat. No. 4,310,721, which is herein incorporated by reference, vocoders are used to analyze the characteristics of a speech signal for encoding, or to synthesize a coded signal and reconstruct an original speech signal. Vocoders implemented with digital signal processor (DSP) technology can be used to greatly reduce the complexity of the vocoder system.
However, the speech coding research resulting in improved vocoder systems has often neglected one specific issue, especially for low-data-rate (e.g., 4.8 kbps and below) vocoder implementations. This issue involves the ability to transfer dual-tone multifrequency (DTMF) signals through the vocoder. The issue is referred to as "DTMF signalling transparency." The issue is significant in that DTMF signalling is required for touch-tone applications in a public-switched telephone network (PSTN) and a private-branch exchange (PBX), as well as, applications such as remote RF signalling, computer access by remote telephone instruments, and many other applications. Since the DTMF signals consist of two simultaneously transmitted audio frequency tones, conventional vocoders have been able to transmit DTMF signals. However, the performance of such vocoders is rather poor, especially in vocoders based on voiced unvoiced characteristics of speech. The poor performance of most conventional vocoders stems from the fact that DTMF signalling transparency has never been a design goal.
The major reason for the lack of design consideration, at least for vocoders operating at 4.8 kbps and less, is that low-data-rate vocoders attempt to exploit speech characteristics (including the linear predictive filter modeling) as much as possible for data compression. As a result, the fidelity of the DTMF signals cannot be well preserved.
Some conventional attempts have been made to provide DTMF signal transparency. For example, one technique involves the use of an external DTMF signal detector 30, as shown in FIG. 3. This detector 30 determines whether or not a DTMF signal appears on an input signal line. If the DTMF signal is detected, the signal line is switched so as to by-pass the vocoder 34. The signal line is fed into a DTMF regenerator 32 so that the detected DTMF signal can then be regenerated. An obvious drawback of this conventional technique is that additional hardware such as the DTMF detector 30 and switching device 33 are required. Additional hardware of this type increases the system cost and requires more space for implementation.
Another technique of providing DTMF signal transparency is to provide the vocoder with a DSP detection program that allows software detection of the DTMF signal within the vocoder. Although this technique eliminates the need for the external hardware required in the previously described conventional technique, previous DSP detection programs were extremely complex and required high computational capabilities. Thus, for many vocoder designs, the conventional DTMF detection schemes were not viable because the vocoders were already designed to run close to the limit of the DSP computing power.